The present invention relates to an electrical switch operated by the input member of a hydraulic master cylinder, such as the master cylinder of a motor vehicle clutch hydraulic control system, the master cylinder of a motor vehicle hydraulic brake system, and the like.
An electrical switch is often associated with the master cylinder of motor vehicle hydraulic brake systems for the purpose of, for example, energizing the electrical circuit turning on the motor vehicle stop signal lights, or/and for operating a cruise control system from an active mode to an inactive mode upon application of the brakes by the driver. Such electrical switches are generally of the pressure operated type. They are operated by the increase of hydraulic fluid pressure when the brake pedal is displaced such as to cause a corresponding displacement of the master cylinder piston, thus increasing the pressure of the hydraulic fluid in the master cylinder in front of the piston. Pressure operated switches are costly to manufacture and they must be installed, at least partly, within the master cylinder. They are often unreliable as they must be activated by a slight increase in the hydraulic fluid pressure for providing an appropriate stop signal even upon slight application of the brakes or for disconnecting the cruise control system as a result of a slight foot tap on the brake pedal.
The functioning of pressure actuated switches may be somewhat erratic under certain conditions, hydraulic fluid back pressure surges in the master cylinder caused, for example, by heat expansion of the brake linings and of the hydraulic fluid. Such erratic functioning of pressure-actuated switches causes erratic turning on and off of the motor vehicle stop signal lights, particularly irritating to the driver of a motor vehicle immediately following.
Hydraulic control apparatus for motor vehicle mechanical clutches are rapidly becoming the standard of the industry for operating the clutch release mechanism of a motor vehicle provided with a mechanical clutch and a conventional manually shiftable transmission or gearbox. Examples of such hydraulic control systems for mechanical clutches are disclosed in U.S. Pat. No. 4,407,125, and in co-pending applications Ser. Nos. 371,958, now abandoned; 376,248, now U.S. Pat. No. 4,599,860; 477,161, now U.S. Pat. No. 4,585,108 ; 477,162, now U.S. Pat. No. 4,585,109; 477,159, now U.S. Pat. No. 4,585,106; 477,160, now U.S. Pat. No. 4,585,107; 537,869, now U.S. Pat. No. 4,684,003; 555,667, now U.S. Pat. No. 4,624,290; 555,666, now U.S. Pat. No. 4,785,615; 555,668, now abandoned all assigned to the same assignee as the present application. It has become general practice in the automobile industry to interlock the operation of the engine starter motor with other controls of the motor vehicle such that the starter motor is rendered inoperative unless the transmission is in neutral or park in motor vehicles provided with an automatic transmission or, in motor vehicles provided with a foot operated clutch and a manually operated gearshift transmission, unless the transmission is in neutral and/or the clutch pedal is fully depressed to fully release the clutch. In addition, it is convenient, in motor vehicles provided with a cruise control and a mechanical clutch, to shut off the operation of the cruise control upon, for example, down shifting which in turn requires release of the clutch. However, it is desirable that the cruise control be disconnected even before the clutch is fully released, to enable the driver to cut off the cruise control by a slight foot tap on the clutch pedal, and perhaps to discourage "riding" the clutch, i.e. resting one's foot on the clutch pedal at all times.